1. Technical Field
Apparatuses and methods consistent with the present invention relate to a method and apparatus for performing ion separation, and more specifically, use of electronic-drive circuits for a differential mobility spectrometer, and a system and method therefore.
2. Related Art
Gas-phase ion separation techniques become increasingly important either as stand-alone systems or in conjunction with mass spectrometry for the analysis of molecular conformations, through chromatographic or mass separations. One way to analyze mixtures of gas phase ions is to move them through a gas by electric fields. The velocity of motion of these ions is proportional to the electrical fields, and the proportionality factor is “ion mobility”, which itself depends on the magnitude of the electric field. If the electric field acts in the direction the ions are moved by the carrier gas, the ions will move through the given length of the electric field a little faster wherby the overall flight times are characteristic of the sizes of the ions and their interactions with the supporting gas atmosphere.
If the electric field acts perpendicularly to the direction in which the ions are moving with the carrier gas, the ions are deflected; the deflection distance is characteristic of the sizes of the ions and their interactions with the supporting gas atmosphere. Accordingly, ions of a particular mobility are separated from ions of all other mobilities present in an initial mixture.
In case one sort of ions has a specific mobility there are two ways to make use of a mobility analysis of a mixture of molecules. A. Finding in a mobility spectrum a peak that is characteristic of a molecule under consideration one has proof that this molecule was part of the initial mixture of molecules. B. Selecting ions that have the mentioned characteristic mobility one can guide these ions to an added analytical device, for instance a mass spectrometer.
Molecule ions of different mobilities can be separated from each other by registering the ion flight time in an accelerating field of given length [1], or by determining the deflection of that ion in an electric field that acts perpendicular to the motion of ions floating in a streaming gas [2].
Separation of ions is based on the differences of the mobilities K(E/N) of these ions varying with the magnitude of the electric field E and the density N of the gas in which the ions are moving. These variations differ for the different sorts of molecules a fact that is used in a “differential mobility spectrometer” [3], [4] in which a high frequency periodic asymmetric waveform of potentials causes, for a short time, a high-field, and for a longer time, a low-field, which forces the ions to oscillate normal to the direction of the carrier gas flow.
The differences in mobility between high and low-field conditions result in a net displacement of the ions, which progressively drift off-axis and eventually discharge on a set of electrodes which can also be used to confine the gas flow. This displacement can be compensated by the application of a dc-field, or alternatively, if a spectrum is required, by scanning a sawtooth-like compensation voltage so that only ions within a certain mobility range are transmitted and recorded consecutively on a Faraday plate.                [1] U.S. Pat. Nos. 3,639,757, 3,697,748, 3,812,355, 3,621,239        [2] U.S. Pat. No. 5,047,723        [3] U.S. Pat. No. 6,774,360; U.S. Pat. No. 6,621,077; U.S. Pat. No. 6,774,360        [4] U.S. Pat. No. 6,495,823 B1, CCCP Patent 966583        [5] Patent Application PCT/US2006/019747, filed May 22, 2006 (H. Wollnik)        
The high-frequency fields in a “differential mobility spectrometer” are formed by applying high-voltage pulses to the electrodes of such a device. In the related art, these pulses are produced by transformer based electronic circuits, in which case the time integral over the positive pulses equals the time integral over the negative pulses, even though the voltages are not constant over the duration of the positive and negative pulses.
In the related art, one uses voltage pulses that have a strong sinusoidal half-wave over the short pulse and a sinus double-wave or triple-wave over the long pulse. As a consequence, in neither one of these pulses do the ions move in a constant field. Thus, the mobilities of the ions vary over the time of each pulse, which in turn compromises the finally achieved separation of ions of different molecules, which in turn comprises the desired resolving power, i.e., the ratio of the width of a peak in the mobility spectrum and the separation of the peak under consideration from other peaks.
Accordingly, it is desirable for the foregoing resolving power to be high, and there is thus a need for a means that improves this resolving power.